It may be jaded to focus on the language of reporting rather than on the victim, but the expression "decapitated head" is jarring and perhaps completely wrong. The victim was decapitated, his body was decapitated, but the head was severed or separated from the body or perhaps "decorporated" (clearly not idiomatic).

My guess is that reporters on CNN, for example, are using the language unthinkingly and might not ever have been taught correct usage. Yet if this is how respected people are using English in public, this usage is or will unfortunately be accepted as correct in the descriptive sense.

It seems to be part of the general laxness such as using "amount" rather than "number" with count nouns.

It's epidemic in some papers. Journalists excuse it against the pressure of deadlines and not having time for careful edits. Yet the text goes through copy editors and at least one additional editor before being printed. In a fast breaking story I can forgive on the spot reporting, but newspapers have at least some time before print journalism happens.